It is a common industrial technique to use computer-controlled and computer-set cutting machines to cut complex shapes into solid metal blanks. One example of a typical and widespread use of this technique is the manufacture of gears, such as spiral and hypoid bevel, spur and helical gears used in automobiles and trucks. After the gears are manufactured they must be deburred to remove excess metal flashing or burrs and may also undergo chamfering to finish the individual gear teeth.
Automated chamfering and deburring machines are known for such relatively simple gear configurations, such as those made and sold by Redin Corporation of Rockford, Ill., Mutschler Technologies, Inc. of North Ridgeville, Ohio and GMI of Independence, Ohio. Other machines have been developed to chamfer and deburr more complex gear shapes such as spiral bevel and hypoid gears. One such machine is made and sold by the Gleason Works of Rochester, N.Y., but can only machine the drive side of each gear tooth. This is acceptable and satisfactory for automotive applications but not for such complex gears such as aerospace-quality spiral and hypoid bevel gears, which require precise bevels on all tooth edges. Such complex aerospace gears are often made in complex geometric shapes and require precision machining to extremely precise tolerances in order to perform satisfactorily. They must also be as lightweight as possible, meaning the gear teeth are made as small as possible yet must withstand maximum load or torque because they are often incorporated in the drive trains of aircraft such as airplanes, helicopters and spacecraft such as the space shuttle.
In an article entitled, “Robotic Automated Deburring of Aerospace Gears,” written by Michael Nanlawala, which appeared in the January/February 2001 issue of Gear Technology magazine, the author describes in detail some of the post-manufacturing processes that are critical to the proper manufacture and performance of complex aerospace gears. In particular, the author states: “Machining processes, such as milling, drilling, turning, hobbing or other gear tooth cutting operations, create burrs on the edges of metal parts when the cutting tool pushes material over an edge rather than cutting cleanly through the material. The size, shape and characteristics of the resulting burrs depend upon a number of process factors, such as tool material and its hardness, tool sharpness, tool geometry, cutting forces, ductility of the material being machined, the speed and feed of the cutting tool, and the depth of cut. A subsequent deburring operation is generally required after those machining processes to remove loose burrs from the machined edge and to apply a chamfer to remove the sharp corners. In addition to the removal of loose burrs, the deburring of the edge produces benefits, such as the removal of sharp edges, increasing the ease of assembly, prevention of edge chipping or breakage, and improvement of air [sic] flow over the edge of rotating parts. Removing sharp edges by deburring and chamfering also eliminates the possibility of stress concentration and increases fatigue life.”
Divots, nicks or cuts formed by uneven or discontinuous chamfering can create stress risers in the manufactured gear which can lead to premature failure of the gear. Heretofore, the most common method of removing burrs and applying chamfers to gear teeth has been the use of manual machining tools applied by a workman to the gear surfaces. Because complex gears have many such surfaces and because the intersection of these surfaces often requires reversal and repositioning of the hand grinder in order to create a continuous chamfer, hand-working such a gear is a laborious and time consuming and relatively imperfect technique. It is not uncommon to have a 30 to 40% rejection rate for hand-chamfered and deburred gears.
As reported by Nanawala, manual deburring has been associated with safety hazards arising from such injuries as cuts, splinters, burns, bruises and eye injuries as well as arthritis, carpal tunnel syndrome and pulmonary illness caused by the inhalation of material ground from the gear blank. Chamfering and deburring a workpiece such as a gear housing can take up to six hours of manual labor with less than reproducibly accurate results.
A typical process used to manufacture aerospace quality gears uses computer controlled or manually set manufacturing or hobbing machines to cut the gear profile from a solid metallic blank. Examples of such a manufacturing tool are the gear cutting machines manufactured by the Gleason Corporation of Rochester, N.Y.
Attempts have been made to use computer-controlled machining tools to perform the chamfering and deburring operations presently being done manually. For example, in the Nanawala article, a number of tools and computer-controlled tool heads are proposed for use in automatic chamfering and deburring operations. None of these combinations of hardware, computer software and tools has proven to be successful on a commercial scale. In particular, Nanawala focuses on the use of force-controlled machining heads. Machining performed by such heads is controlled by the amount of force required to keep the head in contact with the surface or edge to be machined. In order to carry out such an operation, Nanawala describes the use of path programming to teach a computer-controlled robotic machining arm the path required to be followed about the gear periphery in order to carry out the deburring and chamfering operations. Path programming as described by Nanawala uses the “teach pendant” method which requires physically moving the robotic cutting head to a selected point along the gear periphery and recording the position of that point in the computer's memory as well as the orientation of the robotic arm required to machine that point along the gear periphery.
Next, the machining head is moved to a second point and, again, the location of this point as well as the orientation of the machining head is recorded. When a sufficient number of these points are so recorded, it should then theoretically be possible to use the program created to successfully chamfer and deburr the gear. In practice, this trial and error procedure is extremely time-consuming, requires frequent repositioning to adjust the angle of the machining tool to the workpiece and results in wasted workpieces. Moreover, it must be repeated for each gear type whenever a new run of gears is manufactured.
A desirable alternative to the teach pendant technique would be the ability to “model” the gear surfaces in a computer and to use the model to control the machining operation. Computer modeling of complex gears such as spiral bevel aerospace gears is described in an article entitled “New Gear Software” which appears in the January/February 2003 issue of Gear Technology magazine. However, use of the software is limited to creating on-screen solid model depictions of the gears and allowing gear assemblies to be virtually assembled to verify how they fit together. The author of the software is reported as saying: “ . . . he doesn't recommend his latest version for creating the geometry needed to manufacture a spiral bevel gear by traditional metal-cutting methods.” Thus the “New Gear Software” models the gear blank to create a computer image but cannot be used to cut or machine the gear blank.
Examples of the need for chamfering and deburring and attempts at computer-controlled machining operations to carry out these operations are well represented in the prior art.
U.S. Pat. No. 5,091,861 (Geller et al) teaches and describes a system for automatic finishing of machine parts in which the inventors describe the desirability of an automatic, computerized finishing system for machined workpieces and, in particular, state that “Automatic computerized systems for deburring are not known to the inventors.” (col. 1, lines 25-27). This reference teaches the use of solid modeling techniques to deburr straight edges.
U.S. Pat. No. 5,146,670 (Jones) teaches and describes profiling and deburring of workpieces having straight edges.
U.S. Pat. No. 5,675,229 (Thorne) teaches and describes apparatus and methods for adjusting robot positioning which describes in detail the teach pendant method used to generate data sets.
U.S. Pat. No. 6,079,090 (Ongaro) teaches and describes a numeric-control machine tool for turning and hobbing mechanical parts which states at col. 4, lines 26-32 that the apparatus is capable of chamfering but does not describe how the tool is controlled to carry out the chamfering and includes no discussion of complex gear shapes.
U.S. Pat. No. 5,785,771 (Mitchell Jr., et al.) teaches and describes a method for manufacturing precision gears. At col. 1, lines 26-37, the inventors describe with particularity the manufacture of such gears and the advantage to reducing or eliminating the number of scrapped workpieces resulting from said manufacture. At lines 61-64 of col. 1, the inventors confirm the value of the chamfering operation by stating that chamfering reduces stress concentrations in the completed gear.
U.S. Pat. No. 6,074,481 (Bittner, et al.) teaches and describes a masking tool for manufacturing precision gears and method for making same.
U.S. Pat. No. 6,080,349 (Bittner, et al.) teaches and describes a masking tool for manufacturing precision gears and method for making same. This patent is a division of the '41 patent and further particularizes the masking technique described therein.
U.S. Pat. No. 5,810,522 (Parker) teaches and describes a hand-held bar edging tool and support therefor, an example of a hand drill accessory used for hand deburring and chamfering of bar stock.
U.S. Pat. No. 5,154,533 (Baumstark) teaches and describes an apparatus for chamfering and deburring the end edges of a toothed production gear, an example of an apparatus specifically designed to hand-chamfer and deburr the outer edge of a specific gear.
U.S. Pat. No. 4,412,765 (Occhialini) teaches and describes an apparatus for facilitating chamfering/deburring tool and gear meshing, an example of another non-automated apparatus designed to treat specific gears.
U.S. Pat. No. 4,334,810 (Behnke et al) teaches and describes a gear deburring apparatus and method. This reference uses a non-automated drive gear to mesh with the gear workpiece to carry out the machining operation needed to deburr the gear.
U.S. Pat. No. 4,068,558 (Loos) teaches and describes a device for deburring or chamfering of the face edges of gears in which a series of guide discs and cutters specifically designed for each different gear is used to chamfer or deburr the axial edges of the gear teeth.
U.S. Pat. No. 5,960,661 (Massee) teaches and describes an apparatus for a workpiece showing a computer controlled apparatus moveable in two dimensions which does not describe chamfering or deburring.
U.S. Pat. No. 5,901,595 (Massee) teaches and describes an apparatus for machining a workpiece which is a second example of a machining device controlled by a central controller but does not include the capacity for chamfering and deburring.
U.S. Pat. No. 4,565,081 (Massee) teaches and describes a forming machine as yet another example of a memory controlled machining device which does not teach or describe chamfering or deburring.
None of these references teaches nor discloses a method to chamfer and deburr the complex surfaces of a precision spiral gear, together with the capacity to perform other common machining processes such as drilling, honing, reaming, polishing and buffing, nor the apparatus to carry out such a method.